1. Field of the Disclosure
Embodiments disclosed herein relate generally to regeneration of sulfuric acid. More specifically, embodiment disclosed herein relate to extraction of ASO (acid soluble oils) to regenerate spent sulfuric acid. In particular, embodiments disclosed herein relate to using SO2 (sulfur dioxide) to extract ASO from spent sulfuric acid.
2. Background
Concentrated sulfuric acid is a strong Bronsted acid used in a variety of chemical processes to promote acid catalyzed reactions. In particular, sulfuric acid is commonly utilized in alkylation of isoparaffins and olefins for the production of motor fuel alkylate.
In a typical alkylation process, the reaction is carried out in a reactor where the hydrocarbon reactants are dispersed into a continuous acid phase. For example, U.S. Pat. No. 2,762,853 discloses an alkylation process including feeding isoparaffins, such as isobutane or isopentane and C2-C5 monoolefins to an alkylation reactor. The alkylation reaction is catalyzed with sulfuric acid in excess of 88 percent, preferably in excess of 96 percent. The alkylation products are then separated into gasoline range components and heavier alkylate products, among other finishing processes.
As another example, U.S. Pat. No. 2,859,260 discloses an alkylation process including reacting isoparaffins with olefins in the presence of a sulfuric acid catalyst. The reaction product is then separated to recover a hydrocarbon-rich phase and an acid-rich phase. The hydrocarbon-rich phase is further treated to remove catalyst esters from the hydrocarbon phase, among other downstream operations.
All such processes produce a spent acid stream of reduced acid concentration containing a variety of heavy organic compounds that are soluble in the acid, commonly referred to as “acid soluble oils,” or ASO. ASO includes constituents which stay in the sulfuric acid phase upon contacting high strength acid (>80% wt) with hydrocarbon unsaturates. This includes components such as alkyl sulfates (also referred to as mono alkyl sulfates or mono esters), dialkyl sulfate (diesters) which are formed during an alkylation process, and constituents termed conjunct polymers, as described in “Molecular Structure of Conjunct Polymers” by Miron & Lee, Journal of Chemical Engineering, Vol. 8, No. 1, January 1963. Due to recycle and reuse of sulfuric acid in the alkylation process, the concentration of ASO in the spent sulfuric acid builds up over time, requiring regeneration of the spent sulfuric acid.
One process for regenerating spent sulfuric acid streams, as disclosed, for example, in British Patent No. 1,300,835 involves (a) introducing the spent acid into a furnace fired by an air/hydrocarbon fuel mixture wherein the spent acid is converted to sulfur dioxide, water, and carbon dioxide, (b) partially cooling the combustion gas in a waste heat boiler, (c) treating the gas to remove particulates, (d) converting the sulfur dioxide in the gas to sulfur trioxide (by reaction with oxygen in the gas stream) in a fixed bed reactor system containing a vanadium catalyst with or without pretreatment to remove the water contained in the gas stream, and (e) recovering the sulfur trioxide as concentrated sulfuric acid via absorption in dilute sulfuric acid or direct condensation if water is present in the gas stream. The vent gas, after treatment to remove acid mist, generally contains carbon dioxide, oxygen, nitrogen, and a small amount of residual sulfur dioxide.
Other currently known methods for regeneration of sulfuric acid include (a) production of alkyl sulfates or di-alkyl sulfates and their recovery, as disclosed, for example, in U.S. Pat. Nos. 3,534,118; 3,663,648; and 3,673,271; (b) extraction with propane, as disclosed, for example, in U.S. Pat. No. 3,227,775; (c) sulfuric acid crystallization, as disclosed, for example, in U.S. Pat. Nos. 2,716,592 and 2,881,058; and (d) benzene extraction, as disclosed, for example, in U.S. Pat. No. 3,607,035.
For a small operator, the amount of spent acid produced may not be sufficient to justify installation of an onsite conventional acid regeneration facility. Further, sulfuric acid disposal via direct incineration as described above may not be desirable due to increased emissions. The alternative is to ship the spent acid to a large scale sulfuric acid plant serving multiple spent acid generators and to receive purified acid in exchange. However, this alternative may not be particularly attractive when compared to the cost of fresh acid.
Accordingly, there exists a need for additional methods for regenerating spent sulfuric acid that are both economically-feasible and environmentally-acceptable.